Heavy metals and metalloids such as cadmium, lead and mercury are an increasing environmental problem worldwide. Green plants can be used to remove heavy metals by sequestrating, stabilizing or biochemically transforming them. This cost-effective and environment-friendly technology has been called phytoremediation. Hyperaccumulators—heavy metal accumulating flora collected from metal-contaminated sites—offer one option for the phytoremediation of metal-contaminated sites. However, these hyperaccumulators tend to grow slowly and produce little biomass. An alternative approach is to genetically engineer fast-growing species to improve their metal tolerance and metal accumulating capacity.
Selenium (Se) is an essential trace element for animals and bacteria, but is also toxic at higher concentrations. Selenium is naturally present in soils derived from shale rock (up to ˜100 ppm) and when these soils are irrigated, selenate (SeO42−) leaches into the drainage water. In addition, selenite (SeO32−) is a common contaminant in oil refinery wastewater. Plants can take up Se from water, soil or sediment, accumulate it in their tissues, and volatilize it. Volatile forms of Se, such as dimethylselenide (DMSe), have been reported to be 500–600 times less toxic compared to inorganic forms of Se. For Se phytoremediation, terrestrial plants can be grown in Se-contaminated soils, and aquatic plants can be grown in constructed wetlands used for the treatment of Se-contaminated wastewater. The uptake and assimilation of selenate and sulfate are generally assumed to follow the same pathway. Sulfate is actively transported into plant cells by sulfate permease. For reduction, sulfate is first activated by ATP sulfurylase to form adenosine phospho sulfate, which is subsequently reduced to free sulfite by APS reductase. ATP sulfurylase, similar to sulfate permease, was induced by sulfur starvation and repressed by feeding sulfate or reduced forms of S. Here we show that overexpresssion in plants of genes in the sulfate assimilation pathway provides enhanced selenate reduction, Se accumulation and Se tolerance.
We also disclose the surprising finding that overexpression of these genes also enhances heavy metal tolerance and accumulation, and describe fast-growing plants with superior heavy metal accumulation and tolerance for phytoremediation. These transgenic plants overexpressing eznymes from the sulfate assimilation pathway (engineered sulfate assimilator plants) greatly enhance the efficiency of heavy metal phytoextraction from polluted soils and wastewater.
Relevant Literature
See, Pilon-Smits et al., 1999, Plant Physiol 119, 123–32, and references cited therein.